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Programming Arduino
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LED Resistor Calculator
You have to know three things o calculate the resistor:
  • Supply voltage This is how much power you're putting into the circuit. Batteries and wall warts will have the output voltage printed on them somewhere. If you're using multiple batteries*, add the voltage together.
  • LED Voltage Sometimes "Forward Voltage" but usually just abbreviated "V".
    Led Forward Voltage:Amount of voltage that the led requires to light up properly.
    If you don't know the exact voltage rating of the led, you may use the following values as a guide. Note! They may not be the exact values for your leds. Lower voltage is always safer!

    Standard Red: 1.7
    Super Bright Red: 2.2
    Standard Green: 2.2
    High Intensity Blue: 3.0 - 3.5
    High Intensity White: 3.0 - 3.5
  • LED Current Sometimes "Forward Current". This is listed in milliamps or "mA".
    Amout of current that the led uses when powered up properly. 3 mm and 5 mm leds usually operate in the range of 20 - 30 mA. 20 mA is usually a good value to try here. Lumileds and other super bright units can have requirements >200 mA.

Singe LED
Supply voltage volts
Voltage drop accross LED volts
Desired LED current milliamps
Calculated resistor Ohm
Nearest higher rated resistor
LEDs in series
Supply voltage volts
Voltage drop accross LED volts
Desired LED current milliamps
LEDs connected
Calculated resistor Ohm
Nearest higher rated resistor
LEDs in parallell
Supply voltage volts
Voltage drop accross LED volts
Desired LED current milliamps
LEDs connected
Calculated resistor Ohm
Nearest higher rated resistor
ref:
http://www.instructables.com/id/Choosing-The-Resistor-To-Use-With-LEDs/
http://ledz.com/?p=zz.led.resistor.calculator#series